Sera of HBV infected patients, HBV vaccine recipients and HBsAg immunized animals have been examined for their specific reaction to the pre-S gene product. Purified HBsAg is used as the source of material and is subjected to SDS-PAGE electrophoresis and protein blot (Western blot). Polypeptides encoded by the pre-S gene are confirmed by reaction with specific antibodies and their binding to polymerized human albumin. The immunoresponse is studied by its reaction to the specific polypeptides in the protein blots and its serological profile.